Oranic EL array exposure head, imaging system incorporating the same, and array-form exposure head fabrication process
The invention relates to a small-format exposure head wherein ball lenses are positioned in alignment with individual devices of an organic EL array to efficiently collect a light beam from each device onto an image carrier such as a photosensitive material with no crosstalk yet with sufficient resolving power, and an imaging system incorporating the same. An array of organic EL devices is provided on a long length of substrate 3 in at least one row of pixel arrangement. On the light-emitting side of the array of organic EL devices, a ball lens 10 is positioned in alignment and contact with each light emitter 2 of the organic EL device. The ball lens 10 formed of a transparent material having a refractive index of 2 or greater.
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The present invention relates generally to an organic electroluminescent (EL) array exposure head and an imaging system incorporating the same, and more particularly to an organic EL array exposure head adapted to make efficient use of light emitted out of each device of an organic EL array and an imaging system using the same.
The present invention is also concerned with an array-form exposure head fabrication process, especially a process for the fabrication of an organic EL array exposure head, etc. used on an imaging system.
So far, various organic EL array arrangements have been proposed for exposure heads for imaging systems. Pertinent prior arts are given below.
Patent publication 1 proposes that an organic EL array prepared on a glass or other insulating substrate in a batch fashion is combined with a separate driver IC wherein the images of light emitters of the organic EL array are formed on a photosensitive drum by means of a collective rod lens array.
Patent publication 2 proposes the use of a one-chip organic EL array comprising a plurality of rows but discloses nothing about the optical system for forming the images of light emitters on a photosensitive drum. The publication discloses that the EL layer in the organic EL array is deposited by evaporation.
Patent publication 3 teaches that microlenses are formed on the upper surface of a substrate by means of ion exchange or microlenses are prepared by a method wherein a photoresist is applied on the back surface of a substrate or a replica method and an organic EL array comprising resonator structures in alignment with such microlenses is deposited by evaporation.
Patent publication 4 relates to a process for the fabrication of an active matrix type organic display, showing that an organic light-emitting layer is formed on a glass substrate having a thin-film transistor by means of ink jetting.
Patent publication 5 teaches that the hole-injecting layer and organic light-emitting layer of an organic EL device are formed with a bank by means of ink jetting.
Patent publication 6 proposes a printer comprising a photosensitive drum in which a light-emitting layer is provided along with a TFT layer for control of light emission thereof.
Various proposals have also been made with a view to using other arrays such as LED arrays or liquid crystal shutter arrays as exposure heads for imaging systems. In most cases, reliance is on a collective rod lens array for collecting a light beam from the emitter of an LED array or the shutter of a liquid crystal shutter array onto a photosensitive drum.
Patent Publication 1
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- JP-A 10-55890
Patent Publication 2
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- JP-A 11-198433
Patent Publication 3
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- JP-A 2000-77188
Patent Publication 4
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- JP-A 10-12377
Patent Publication 5
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- JP-A 2000-323276
Patent Publication 6
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- JP-A 2001-18441
Patent Publication 7
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- JP-A 2000-353594
Non-Patent Publication 1
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- “Polymer Type Organic EL Display”, The 8th Electronic Display Forum (Apr. 18, 2001)
Non-Patent Publication 2
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- “Fine Imaging and Hardcopy”, edited by the Photography Society of Japan, the Joint Publication Committee of the Image Society of Japan, Corona Publisher Co., Ltd., Jan. 7, 1999, page 43
In the above prior art wherein an organic EL array is used on the exposure head of an electrophotographic type printer, the use of a collective rod lens array for collecting a light beam from the emitter of an organic EL device in the organic EL array onto a photosensitive drum incurs an increase in the length of an optical path, resulting in an increase of the size of the arrangement. The use of the collective rod lens, because of being not located in one-to-one relation to each emitter, gives rise a periodical variation in the quantity of light. In addition, the collective rod lens is unavoidably expensive because high techniques are needed for its fabrication. A problem with microlenses is that crosstalk is likely to occur as a result of light that emerges from a specific emitter but is incident on an unassociated pixel position via a microlens adjacent thereto, not via the associated microlens, leading to a resolving power drop.
In an arrangement wherein substrates are applied to individual microlenses one by one, some problems arise with the mechanical strength required for maintenance operations such as that for removal for toner deposited to an optical system.
On the other hand, high-refractive-index ball lenses used for reflective paints, etc. available for enhancing the visibility of traffic signs at nighttime are very expensive, and so are expected to provide an optical system at very low costs if they can be used for array-form exposure heads such as organic EL array heads.
SUMMARY OF THE INVENTIONIn view of such problems with the prior art as mentioned above, one object of the invention is to provide a small-format exposure head comprising ball lenses located in alignment with individual devices of an organic EL array so that a light beam from each device can be efficiently collected onto an image carrier such as a photosensitive member with no crosstalk yet with sufficient resolving power, and an imaging system incorporating the same.
Another object of the invention is to provide a small-format exposure head comprising ball lenses firmly fixed in alignment with individual devices of an organic EL array so that a light beam from each device can be efficiently collected onto an image carrier such as a photosensitive member with no crosstalk yet with sufficient resolving power and sufficient mechanical strength is ensured for maintenance operations, and an imaging system incorporating the same.
Yet another object of the invention is to provide an organic EL array exposure head comprising ball lenses located in alignment with individual devices of an organic EL array, wherein possible poor exposure due to fouling by toner is prevented.
A further object of the invention is to provide a process for the fabrication of an array-form exposure head comprising ball lenses fixed in alignment with individual devices of array-form light sources in an organic EL array, an LED array or the like, so that a light beam from each device can be efficiently collected onto an image carrier such as a photosensitive member with no crosstalk yet with sufficient resolving power.
According to the first aspect of the invention, the above objects are achievable by the provision of an organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of said array of organic electroluminescent devices, characterized in that:
said ball lens is located in such a way as to satisfy condition (8):
0≦D≦0.07 m (8)
where D is the distance between the surface of a light-emitting layer in the light emitter of each organic electroluminescent device and the apex of the entrance side-surface of the ball lens positioned in alignment with said light emitter, and m is the diameter of the ball lens.
Preferably in this case, the ball lens is formed of a transparent material having a refractive index of 2 or greater.
According to the second aspect of the invention, there is provided an organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of the array of organic electroluminescent devices while said ball lens is in contact with said light emitter, characterized in that said ball lens is formed of a transparent material having a refractive index of 2 or greater.
Preferably, the arrangement according to the first or second aspect of the invention comprises an array of a plurality of rows of organic electroluminescent devices parallel with one another, wherein a light emitter of each organic electroluminescent device in one row is displaced from a light emitter of each organic electroluminescent device in another row by 1/number of rows with respect to a spacing between adjacent light emitters in a row.
The above arrangement may further comprise at least two array sets in a columnar direction, wherein each array set comprises a plurality of rows of organic electroluminescent devices.
Preferably in this arrangement, a light absorber is filled between adjacent ball lenses an area except an effective exit surface.
According to the third aspect of the invention, there is provided an organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of the array of organic electroluminescent devices, characterized in that:
a space between adjacent ball lenses is filled with a fixing layer up to a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, wherein m is the diameter of said ball lens and n is the refractive index of said ball lens, so that said ball lens is fixed in place.
Preferably in this case, that fixing layer comprises a layer capable of absorbing light.
According to the fourth aspect of the invention, there is provided an organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of the array of organic electroluminescent devices, characterized in that:
said ball lens is at least embedded in an aerogel layer from a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, where m is the diameter of said ball lens and n is the refractive index of said ball lens, up to a height at which the exit side-surface of said ball lens is overall covered with said aerogel layer.
Preferably in this case, a transparent protective layer is provided on the surface of said aerogel layer, and the transparent protective layer comprises a transparent conductive layer.
A fixing layer capable of absorbing light may further be provided on a substrate side of said aerogel layer.
In the organic electroluminescent array exposure head according to the third or fourth aspect of the invention, it is preferable that the ball lens has a refractive index of 2 or greater.
The present invention also includes an imaging system that incorporates any one of such organic electroluminescent array exposure heads as recited above as an exposure head for writing an image to an image carrier. A typical imaging system of the invention is a tandem type color imaging system comprising at least two imaging stations, each comprising around the image carrier a charger means, an exposure head, a toner developer means and a transfer means, wherein color imaging is carried out by passing a transfer medium through each station.
The first organic EL array exposure head of the invention comprises a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of said array of organic electroluminescent devices, wherein said ball lens is located in such a way as to satisfy condition (8):
0≦D≦0.07 m (8)
where D is the distance between the surface of a light-emitting layer in the light emitter of each organic electroluminescent device and the apex of the entrance side-surface of the ball lens positioned in alignment with said light emitter, and m is the diameter of the ball lens. Thus, nearly all diverging light leaving the light emitter of each organic EL device can be entered in the ball lens so that it can be utilized for pixel formation by projection; images of sufficient resolving power, brightness and contrast can be obtained at low power consumption yet with little or no crosstalk between adjacent pixels.
The third organic EL array exposure head of the invention comprises a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of the array of organic electroluminescent devices, wherein a space between adjacent ball lenses is filled with a fixing layer up to a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, wherein m is the diameter of said ball lens and n is the refractive index of said ball lens, so that said ball lens is fixed in place. Thus, the ball lenses can be firmly fixed onto the substrate with the light emitters located thereon without cutting off a light beam that leaves the light emitter and is projected onto a given surface to form a pixel, thereby obtaining a small-format organic EL array exposure head having sufficient mechanical strength for maintenance operations. By using a layer capable of absorbing light as that fixing layer, it is further possible to achieve an organic EL array exposure head that has reduced crosstalk between adjacent pixels and can give an image of sufficient resolving power, brightness and contrast at low power consumption.
In the fourth organic EL array exposure head of the invention, the ball lens is at least embedded in an aerogel layer from a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, where m is the diameter of said ball lens and n is the refractive index of said ball lens, up to a height at which the exit side-surface of said ball lens is overall covered with said aerogel layer. Thus, the ball lenses can be firmly fixed onto the substrate with the light emitters located thereon without cutting off a light beam that leaves the light emitter and is projected onto a given surface to form a pixel, thereby obtaining a small-format organic EL array exposure head having sufficient mechanical strength for maintenance operations. By providing a transparent protective layer, especially a transparent protective layer comprising a transparent conductive layer on the surface of the aerogel layer, it is possible to prevent fouling of the optical system of the organic EL array exposure head with toner or the like, which may otherwise lead to poor exposure or other defects.
To attain the aforesaid objects, the present invention further provides a process for fabricating an array-form exposure head comprising a long length of substrate, an array of light-emitting devices or secondary light sources arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each light-emitting device or each secondary light source on the light-emitting side of the array of light-emitting devices or secondary light sources, characterized in that:
a ball lens alignment mold is provided with an array of ball-receiving holes in an arrangement conforming to an arrangement of light emitters of said array of light emitting devices or secondary light sources, wherein each ball-receiving hole has a diameter equal to or slightly larger than that of said ball lens,
ball lenses are fitted in said ball lens-receiving holes in said ball lens alignment mold, and
each ball lens on said ball lens alignment mold is brought close to each light emitter of said array of light-emitting devices or secondary light sources while in alignment therewith, whereupon each ball lens is bonded to said array of light-emitting devices or secondary light sources.
Preferably in this case, each ball lens-receiving hole in said ball lens alignment mold is provided with a ball lens suction mechanism for facilitating fitting of the ball lens in each ball lens-receiving hole and retaining the ball lens fitted in each ball lens-receiving hole until the ball lens is bonded and fixed to said array of light-emitting devices or secondary light sources.
It is desired that prior to bringing each ball lens on said ball lens alignment mold close to each light emitter of said array of light emitting devices or secondary light sources while in alignment therewith, whether or not a given ball lens is fitted in each ball lens-receiving hole in said ball lens alignment mold is inspected.
It is also desired that a light-emitting device is placed at the center of the bottom of each of said ball lens-receiving holes, and whether or not a given ball lens is fitted in each of said ball lens-receiving holes in said ball lens alignment mold is inspected from the light-quantity profile found when said light-emitting devices are all allowed to emit light.
It is further desired that prior to bringing each ball on said ball lens alignment mold close to each light emitter of said array of light-emitting devices or secondary light sources while in alignment therewith, an adhesive agent is applied on a surface of the substrate of said array of light-emitting devices or secondary light sources or portions of said ball lens opposite to said surface, whereupon each ball lens is brought close to and bonded to said array of light-emitting devices or secondary light sources.
Furthermore, it is desired that each ball lens is brought close to each light emitter of the array of light-emitting devices or secondary light sources, and after bonded to that array, a fixing layer is filled between adjacent ball lenses at an area except an effective exit surface.
For instance, an organic EL array or an LED array may be used as the above array of light-emitting devices or secondary light sources.
The present invention also includes an imaging system incorporating an array-form exposure head fabricated by the above fabrication process as an exposure head for writing an image to an image carrier. A typical imaging system of the invention is a tandem type color imaging system comprising at least two imaging stations, each comprising around the image carrier a charger means, an exposure head, a toner developer means and a transfer means, wherein color imaging is carried out by passing a transfer medium through each station.
In the above array-form exposure head fabrication process of the invention, a ball lens alignment mold is provided with an array of ball-receiving holes in an arrangement conforming to an arrangement of light emitters of said array of light emitting devices or secondary light sources, wherein each ball-receiving hole has a diameter equal to or slightly larger than that of said ball lens, ball lenses are fitted in said ball lens-receiving holes in said ball lens alignment mold, and each ball lens on said ball lens alignment mold is brought close to each light emitter of said array of light-emitting devices or secondary light sources while in alignment therewith, whereupon each ball lens is bonded to said array of light-emitting devices or secondary light sources. It is thus possible to make use of inexpensive, high-refractive-index ball lenses used for reflective paints or the like to fabricate an array-form exposure head such as an LED array exposure head at extremely low costs. In addition, it is possible to achieve an automated optical system alignment/fixation system for high-quality, low-cost array-form exposure heads, thereby fabricating high-quality, low-cost array-form exposure heads.
Further, no large impact is likely to be applied on the ball lenses upon alignment, and so the ball lenses are unlikely to chip off to ensure provision of a high-quality array-form exposure head.
Furthermore, whether alignment is acceptable or unacceptable, and whether or not ball lenses chip off can be 100% inspected at high speed and with high precisions, so that high-quality, low-cost array-form exposure heads can be provided.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
Preferred embodiments of the organic EL array exposure head according to the invention and the imaging system incorporating the same are now explained with reference to the accompanying drawings.
Each ball 10 should be located in contact with or near to the surface of the associated light emitter 2, so that nearly all of divergent light leaving the light emitter 2 can enter the associated ball lens 10. It is here noted that if the diameter s of the light emitter 2 is much lager than the diameter m of the ball lens 10 and the ball lens 10 is in contact with the surface of the light emitter 2, almost 100% of the light can then enter the ball lens 10. Thus, it is possible to reduce cross-talk—a phenomenon wherein the light emerging from the light emitter 2 is incident on an unassociated pixel position via a ball lens 10 adjacent to the associated ball lens 10.
To allow a light beam leaving the ball lens 10 located in contact with or near to the surface of the light emitter 2 to be split as a pixel for incidence on the image carrier 11, the ball lens 10 should be formed of a transparent material having a refractive index of at least 2 with respect to light emitted out of the light emitter 2. Glass materials having a refractive index of 2 or more, for instance, include bismuth-based glasses, glasses containing much heavy metals or optical crystals.
Important geometric optics characteristics of the ball lens 10 having a refractive index of 2 or more are now explained. The focal length f of the paraxial optical ball lens 10 is determined by
f=nr/{2(n−1)} (1)
with a principal point in agreement with the center of the ball lens 10. When the refractive index n is n<2, the focus F is located outside of the ball lens 10 as shown in
With the ball lens 10 of n≧2 located in contact with or near to the surface of the light emitter 2 as shown in
With such reduced crosstalk as mentioned above in mind, it is desired that the ball lenses 10 be as near to the light emitters 2 of the organic EL array as possible, i.e., the distance D between the apex of the entrance side surface of each ball lens 10 and the surface of the associated light emitter 2 be D≈0. Because a transparent electrode or the like is actually present on the surface of the light emitter 2, however, the distance D will unavoidably have a certain value. To what length is acceptable for the distance D is now considered.
With an apparent light source B positioned at a point B spaced δ-away form a ball lens 10, as shown in
At this time, the angle θ may be represented by
sin θ=r/(r+δ) (2)
where r is the radius of the ball lens 10.
Suppose now that the ball lens 10 is bonded onto a substrate 18 having a refractive index n and a light source is positioned at a point A within the substrate 18. Then, crosstalk could occur because, in
The angle β is then represented by
sin β=r/{n(r+δ)} (3)
Next, of the light emerging from the point A, the quantity of light that can be extracted out of the substrate (organic EL device) 18 is found. By virtue of the presence of a total reflection critical angle θc due to the refractive index of the substrate 18, light propagating from within the substrate 18 at the exit angle β toward the surface of the substrate 18 is taken out of the substrate (organic EL device) 18 provided that θc>β. The emission intensity of a surface light emitter harnessing isotropic light emission such as the light emitter 2 of the organic EL device conforms to Lambert's law. Given I(θ)=I0 cos θ, the quantity of light radiated within the critical angle becomes
Here dΩ is a solid angle.
Of the light radiated from the point (light source) A within the critical angle θc, the quantity of light that does not enter the ball lens 10 is given by
Therefore, of the light emerging from each light emitter 2 of the organic EL array 1, the proportion of the quantity of light that could become crosstalk is represented by
(2rδ+δ2)/(r+δ)2 (6)
Here, if it is acceptable that up to 5% of the emerging light become crosstalk, the following condition is obtained.
δ≦0.026 r (7)
When, at δ=0.026 r, the refractive index n of the organic EL array 1 (the refractive index of the substrate 18) is set at 1.5, the distance between the points A and L in
From the above consideration, it is appreciated that the distance D between the surface of each light emitter 2 of the organic EL array 1 and the apex of the entrance side surface of the ball lens 10 located in line with each light emitter 2 should preferably satisfy the following relation:
0≦D≦0.07 m (8)
Here m is the diameter of the ball lens 10 as described above.
In the aforesaid organic EL array exposure head of the invention, the ball lenses 10 are positioned in contact with or near to the surfaces of the light emitters 2 in one-to-one relations. This will inevitably make the image-side conjugate distance (from the ball lens 10 to the image carrier 11) longer than the object-side conjugate distance; the image 2′ of each light emitter 2 projected onto the image carrier 11 will be magnified. To allow the images 2′ of the light emitters 2 to be mutually separated upon projection onto the image carrier 11 with the desired resolution, the dimension s of the light emitters 2 must be much smaller than the diameter m of the ball lenses 10 in such a way that the magnification can be compensated for. With the dimension s of the light emitters 2 relatively reduced while taking such relations in account, the proportion of divergent light leaving the light emitters 2 and entering the ball lenses 10 becomes so high that crosstalk can be cut off.
When it comes to an exposure head of 600 dpi as a specific example, the distance between the ball lens 10 and the image carrier 11 should be 300 μm and the desired size of one pixel (the size of the image 2′) on the image carrier 11 should be □70 μm. In other words, when the ball lens 10 selected has a refractive index of n=1.5 and a diameter of m=120 μm, the size of the organic EL light emitter 2 should be □20 μm.
As can be understood from the foregoing, the diameter m of the ball lens must be increased to make the image 2′ of the light emitter 2 resolvable on the surface onto which the image is to be projected (the surface of the image carrier 11) and reduce crosstalk. With the organic EL array comprising one row of light emitters 2, however, it is difficult to achieve an exposure head having the desired resolution, because the pitch p between the ball lenses 10 can be never smaller than the diameter m thereof. In this case, such an exposure head having the desired resolution can be achieved by allowing the organic EL array 1 to have a plurality of parallel rows (e.g., three rows 201, 202 and 203) of light emitters 2 and locating ball lenses 10 in alignment with the light emitters in three such rows 201, 202 and 203 and in one-to-one relations, as shown typically in
For instance, suppose now that the desired resolution is 600 dpi and the diameter of the ball lens 10 is 120 μm. Then, the resolution at a pitch given by the diameter of the ball lens 10 of 120 μm, i.e., the resolution for only the light emitters 2 in any one of the rows 201, 202 and 203 is 211.7 dpi. Thus, if the pitch of the light emitters 2 in any one of the rows 201, 202 and 203 is determined in such a way as to be smaller than 211.7 dpi, e.g., 200 dpi that is the maximum among the submultiples of 600, say, 200 (a pitch of 127 μm), the desired resolution can then be achieved by subjecting an exposure head to scanning exposure with respect the image carrier 11 in a direction indicated by a double-action arrow while the rows 201, 202 and 203 of the light emitters 2 are displaced by a pitch for the desired resolution (42.33 μm that is a pitch of 600 dpi) in a row direction and arranged in the number corresponding to (the desired resolution (dpi)/resolution (dpi) for one row) (e.g., three rows 201, 202 and 203) in a columnar direction.
It is noted that no particular limitation is imposed on the pitch between the rows 201, 202 and 203 in the columnar direction; in the embodiment of
In this connection, when the quantity of light exposure to the surface onto which an image is to be projected (the surface of the image carrier 11) is insufficient for one pixel that is the image 2′ of the light emitter 2 and the photosensitization of the image carrier 1 by one light emission of the light emitter 2 is insufficient, an exposure head may be set up by the juxtaposition in the columnar direction of sets of rows 201, 202 and 203 of
When the size s of each light emitter of the organic EL array 1 is reduced as described above, the quantity of light utilized for exposure (the efficiency of utilization) is now considered.
Suppose now that a light-emitting point A exists within a substrate (organic EL device) 18 having a refractive index n. By virtue of the presence of a total reflection critical angle θc due to the refractive index of the substrate 18, light propagating from within the device at an exit angle θ toward an interface is taken out of the device provided that θc>θ. The emission intensity of a surface light emitter using isotropic light emission such as an organic EL device conforms to Lambert's law. Given I(θ)=I0 cos θ, integration with respect to the solid angle of an upper half gives
Here Ω is a solid angle. Division of the quantity of light radiated within the critical angle θc by the total quantity of light πI0 gives
Therefore, the efficiency of light extraction of the organic EL device itself is represented by 1/n2.
If the refractive index of the device substrate is n=1.5, the efficiency of light extraction is then calculated to be 44.4%.
When a collective rod lens array is used as a projection optical system for the projection of the image of the light emitter 2 of the organic EL onto the image carrier 11 as in the prior art, the efficiency of utilization of light of the optical system is found by multiplying the above calculated value of 44.4% by a collection coefficient of 7% that is substantially the collection coefficient of the collective rod lens array, say, 0.444×0.07≈0.031. In other words, 3.1% of the total quantity of light emitted out of the organic EL device are utilized to form an image on the image carrier 11.
On the other hand, when the ball lens 10 is located in contact with or near to the surface of the light emitter 2 as in the invention, it is possible to collect 30% of the intensity of light from the surface of the light emitter 2 onto the image carrier 11, as calculated by ray tracing.
It is thus possible to achieve the efficiency of utilization of about ten times as high as that achieved with the collective rod lens array.
It is here noted that a collective rod lens array provides an optical system of 1× magnification wherein the size of the light emitter 2 is substantially the same as the size of an image formed on the image carrier 11, whereas the exposure head of the invention takes the form of a magnifying optical system wherein, as described above, the size of the light emitter 2 must be more reduced than the pixel size on the image carrier 11 by factor of the magnifying rate. However, if the light emitter 2 used is of the same luminance, the area of the light emitter 2 can then be reduced to 1/10 (a magnifying rate of 3.2) as compared with the collective rod lens array while the same light intensity is maintained of the pixels on the image carrier 11. It is thus possible to achieve some considerable reductions in the power consumed by the exposure head.
The present invention is now explained with reference to
An organic EL device is generally designed to extract light from either a cathode or an anode. In this embodiment, the light is extracted from the cathode side. More specifically, the surface of a substrate 3 formed of glass, silicon or the like is provided at a given pitch with a row of holes 4 for receiving light emitters of the organic EL device. On the bottom of each hole 4, an anode 21 wherein an ITO or the like is laminated on a reflective metal film formed of a magnesium/silver alloy, aluminum or the like, a hole-injecting layer 22 and a light-emitting layer 23 are stacked in this order. A transparent cathode 24 that is thin enough to transmit light is provided all over the surface of the substrate 3 in such a way as to be in contact with the light-emitting layers 23 in the holes. A light emitter 2 corresponding to each hole 4 has a structure wherein the light-emitting layer 23 capable of transporting electrons and the hole-injecting layer 22 are stacked one upon another, so that electrons and holes injected from the transparent cathode 24 and anode 21, respectively, can be recombined to emit light.
A sealing layer 25 is provided on the transparent cathode 24 for prevention degradation of the light-emitting layer 23 by touch with moisture, and an adhesive layer 26 formed of transparent resin is provided thereon to fix ball lenses 10 in alignment with the respective light emitters 2 with respect to the substrate 3. A black resin 27 is filled between adjacent ball lenses 10 at an area except an effective surface from which a projection light beam 12 is to emerge, thereby absorbing noise light such as crosstalk light occurring as a result of a light beam that emerges from a specific light emitter 2 but is incident on an unassociated pixel position via a ball lens 10 adjacent to the associated ball lens 10, and flare light occurring as a result of repeated total reflections of the light beam within the ball lens 10.
Referring to
The second aspect of the organic EL array exposure head according to the invention is now explained.
Each ball 10 should be positioned in contact with or near to the surface of the associated light emitter 2, so that nearly all of divergent light leaving the light emitter 2 can enter the associated ball lens 10. It is here noted that if the diameter s of the light emitter 2 is much lager than the diameter m of the ball lens 10 and the ball lens 10 is in contact with the surface of the light emitter 2, almost 100% of the light can then enter the ball lens 10. Thus, it is possible to reduce cross-talk—a phenomenon wherein the light emerging from the light emitter 2 is incident on an unassociated pixel position via a ball lens 10 adjacent to the associated ball lens 10.
The ball lenses 10 positioned in one-to-one relations with the emitters 2 of the organic EL array 1 are bonded to the substrate 3 through a transparent adhesive layer 13. In the embodiment of
The thickness d of the ball lens 10 from the surface apex on the entrance side to the upper surface of the fixing layer 14 depends on the refractive index of the ball lens 10. Typically, when the refractive index of the ball lens 10 is n=2, the thickness d is set at 0.75 m where m is the diameter of the ball lens 10, and at n=2.2, the thickness d is set at about 0.8 m.
What is meant by the thickness d is now explained. Presume now that, as shown in
As shown in
∠ORL=∠OLR=θ(∵OR=OL) (11)
∠BRL=∠OLR=θ(∵OL is parallel with BR) (12)
∵∠ORB=∠OLR+∠BRL=2θ (13)
Let m be the diameter of the ball lens 10. Then,
BR=OR cos 2θ=(m/2)cos 2θ (14)
AB=(m/2) (15)
Therefore, the height AR of the point R becomes
AR=AB+BR=(m/2)(1+cos 2θ) (16)
Let α be the critical angle between the ball lens 10 and the air interface and n be the refractive index of the ball lens 10. Then,
sin α=1/n (17)
Hence,
cos 2α=cos2 α−sin2 α=1−2 sin2 α=1−2/n2 (18)
At θ≦α, that light ray leaves the ball lens 10. However, substituting θ≦α into equation (16) gives
Ar≧(m/2)(1+cos 2α)=m(1−1/n2) (19)
This means that the light ray can emerge from only a site of height that satisfies equation (19) concerning the ball lens 10, and that even when other sites are covered with the fixing layer 14 formed of a black resin or the like, there is no detriment to the function of the ball lens 10.
Suppose here that at n=2, the thickness d of the lens ball 10 up to the upper surface of the fixing layer 14 is d≦0.75 m, and at n=2.2, d=0.793 m≈0.8 m or less. Then, the ball lens 10 can be firmly fixed onto the light emitter 2 of the organic EL array 1 without cutting off the light beam 12 emerging form the light emitter 2, which is projected onto the image carrier 11 to form a pixel thereon.
The fixing layer 14 is not necessarily formed of a transparent material. In consideration of possible noise light such as crosstalk light occurring as a result of a light beam that emerges from a specific light emitter 2 but is incident on an unassociated pixel position via a ball lens 10 adjacent to the associated ball lens 10, and flare light occurring as a result of repeated total reflections of the light beam within the ball lens 10, however, it is preferable to form the fixing layer 14, using a light-absorbable black resin or the like.
To allow light beams leaving the ball lenses 10 located in contact with or near to the surfaces of the light emitters 2 to be split as pixels for incidence on the image carrier 11, the ball lens 10 should be formed of a transparent material having a refractive index of at least 2 with respect to light emitted out of the light emitter 2. Glass materials having a refractive index of 2 or more, for instance, include bismuth-based glasses, glasses containing much heavy metals or optical crystals.
When ball lenses 10 having a refractive index of 2 or greater are positioned in contact with or near to the surfaces of light emitters 2, light beams 12 leaving the light emitters 2 via the ball lenses 10 are split into parallel or converging light rays for incidence on the image carrier 11, as explained with reference to
For the purpose of reducing crosstalk as mentioned above, it is still desirable to satisfy the relation defined by equation (8) as described with reference to
It is here noted that when a transparent material having a refractive index close to 1 is selected for the material for the fixing layer, equation (19) concerning height is not necessarily satisfied; the fixing layer may be filled between adjacent ball lenses 10 in such a way that the ball lenses 10 are embedded therein. Among such transparent materials having a refractive index close to 1, there is aerogel (for instance, see non-patent publication 1). One typical aerogel is silica aerogel. An example of using the silica aerogel for the fixing layer is shown in
However, a transparent protective layer 16 is provided over the surface of the silica aerogel layer 15. The reasons are that silica aerogel has a very high porosity, and so when the silica aerogel layer 15 remains unprotected, it may possibly take up moisture in the air to such an extent that the refractive index or other physical properties change or toner or other particles may possibly enter surface pores too deeply to remove them. The transparent protective layer 16 is formed of ITO or other transparent conductive layer. In addition, if the transparent conductive layer is controlled in such a way as to have the same potential as that of a toner in a developer in an electrophotographical system, it is then possible to prevent fouling of the optical system in the organic EL array exposure head 1 with toner particles, which may otherwise lead to poor exposure or other defects.
Regarding the quantity of light (efficiency of utilization) used for exposure where the size s of the light emitters 2 of the organic EL array 1 is reduced, comparisons with where a collective rod lens array is used as a projection optical system for the projection of the light emitters 2 onto the image carrier 11, etc., see the explanation made with reference to
The present invention is now explained with reference to
An organic EL device is generally designed to extract light from either a cathode or an anode. In this embodiment, the light is extracted from the cathode side. More specifically, the surface of a substrate 3 formed of glass, silicon or the like is provided at a given pitch with a row of holes 4 for receiving light emitters of the organic EL device. On the bottom of each hole 4, an anode 21 wherein an ITO or the like is laminated on a reflective metal film formed of a magnesium/silver alloy, aluminum or the like, a hole-injecting layer 22 and a light-emitting layer 23 are stacked in this order. A transparent cathode 24 that is thin enough to transmit light is provided all over the surface of the substrate 3 in such a way as to be in contact with the light-emitting layers 23 in the holes. A light emitter 2 corresponding to each hole 4 has a structure wherein the light-emitting layer 23 capable of transporting electrons and the hole-injecting layer 22 are stacked one upon another, so that electrons and holes injected from the transparent cathode 24 and anode 21, respectively, can be recombined to emit light.
A sealing layer 25 is provided on the transparent cathode 24 for prevention degradation of the light-emitting layer 23 by touch with moisture, and an adhesive layer 13 formed of a transparent resin is provided thereon to fix ball lenses 10 in alignment with the respective light emitters 2 to the substrate 3. A fixing layer 14 formed of a black resin is filled between adjacent ball lenses 10 at an area except an effective surface from which a projection light beam 12 is to emerge, thereby fixing all the ball lenses 10 firmly onto the light emitters 2 of the organic EL array 1. This fixing layer 14 then serves to absorb noise light such as crosstalk light occurring as a result of a light beam that emerges from a specific light emitter 2 but is incident on an unassociated pixel position via a ball lens 10 adjacent to the associated ball lens 10, and flare light occurring as a result of repeated total reflections of the light beam within the ball lens 10.
In the embodiment of
The present invention is now explained with reference to specific embodiments of fabricating an array-form exposure head wherein, as is the case with the above organic EL array, ball lenses are positioned in alignment with light emitters of an array-form light source. It is noted that these embodiments may be applied to an array-form light source such an LED array, let alone the organic EL array.
As shown in
Then, as shown in
In the step of
In a step of
In the step of
Alternatively, a light quantity profile pattern on the surface of the ball lens alignment mold 80 or a surface at a given distance from that surface may be subject to image processing while all the light emitters 82 at the bottoms of the ball lens-receiving holes 81 are allowed to emit light, thereby making a judgment of whether or not the ball lenses 10 are put in order in the step of
Further, in place of locating the light emitter 82 at the bottom of each ball lens-receiving hole 81, it is acceptable to form the ball lens alignment mold 80 using a transparent material or form a minute through hole in the center of the bottom of each ball lens-receiving hole 81. In either case, the ball lens alignment mold 80 is illuminated from its back surface with parallel light to make detection of light quantity with the above inspection jig 87 or subject a light quantity profile pattern to image processing for inspection.
Furthermore, it is acceptable to take an image of the ball lens alignment mold 80 from above, so that the image can be subject to image processing, thereby making an inspection of whether or not the given ball lenses 10 are fitted in all the ball lens-receiving holes 81.
Once the lens ball alignment mold has been judged as being acceptable (OK) in the inspection step of
In the step of
Once the transparent adhesive layer 13 has been cured, negative pressures in ball lens-receiving holes 81 in the ball lens alignment mold 80 are cut off to detach the ball lens alignment mold 80 from the organic EL array 1 with the ball lenses 10 bonded thereto, as shown in
It is noted that after the inspection step of
The array-form exposure head may take a form wherein, for instance, ball lenses 10 are bonded and fixed to emitters 2 of an array-form light source 1 in one-to-one relations, as shown in
One embodiment of filling the fixing layer 14 formed of a black resin or the like between adjacent ball lenses 10 at an area except an effective surface from which the projection light beam 12 is to emerge is now explained with reference to
How to fabricate the organic EL array 1 used in the embodiment of
Then, a bank 33 having a given height is formed with a hole 4 corresponding to the organic EL light emitter 2. As set forth in patent publication 5, the bank 33 may be prepared by any desired method inclusive of photo-lithography or printing. For instance, when photolithography is used, a coating of an organic material is formed with a bank height by means of given techniques such as spin coating, spray coating, roll coating, die coating or dip coating, followed by application of a resist layer thereon. Then, a mask conforming to the shape of the bank 33 is provided to expose and develop the resist, leaving a resist conforming to the shape of the bank 33. Finally, the bank material is etched to remove a bank material portion other than the mask. Alternatively, the bank (protruding portion) may be comprised of at least two layers wherein the lower layer is formed of an inorganic material and the upper layer is formed of an organic material. As set forth in patent publication 5, the material used to form the bank 33 may be selected from any desired material having sustained resistance to solvents for EL materials; however, it is preferable to employ organic materials such as acryl resins, epoxy resins and photosensitive polyimides because they can be converted into Teflon by fluorocarbon gas plasma treatments. It is also acceptable to make use of a multilayered bank wherein the lower layer is formed of an inorganic material such as liquid glass. The bank 33 should preferably be blackened or opaqued by incorporation of carbon black or the like in the above materials.
Then, the substrate with the bank 33 formed thereon is continuously plasma treated with oxygen gas and fluorocarbon gas plasma just prior to coating of an ink composition for an organic EL light-emitting layer. Thus, for instance, the surface of the polyimide forming the bank 33 is made water-repellent while the surface of the anode 21 is rendered hydrophilic, so that the wettability of the substrate side for fine patterning of ink jet droplets can be controlled. The plasma generator used may be either of the type that generates plasma in vacuum or of the type that generates plasma in the air.
Then, an ink composition for the hole-injecting layer is jetted into a hole 4 in the bank 33 through a head 71 of an ink jet type printer 70 for patterning coating on the anode 21 of each pixel. After coating, the solvent is removed and thermal treatment is carried out to form a hole-injecting layer 22.
It is here noted that the “ink jet type” referred to herein may be either a piezo-jet type wherein an ink composition is jetted by making use of mechanical energy of a piezoelectric device or the like or a thermal type wherein an ink composition is jetted as a result of formation of air bubbles by thermal energy of a heater (cf. non-patent publication 3). A typical arrangement of a head for the piezo-jet type is shown in
After a hole-injecting layer 22 is formed on an anode 21 in a hole 4, the ink composition for the light-emitting layer is jetted out of the head 71 of the ink jet printer 70 onto the hole-injecting layer 22 in the hole 4 for patterning coating on the hole-injecting layer 22 of each pixel. After coating, the solvent is removed and thermal treatment is carried out to form a light-emitting layer 23.
It is here noted that the above order of the light-emitting layer 23 and hole-injecting layer 22 may be reversed. Preferably in this case, a layer more resistant to moisture is located on the surface side (that faces away from the substrate 3).
The hole-injecting layer 22 and light-emitting layer 23 may also be prepared by using known techniques such as spine coating, dipping or evaporation in place of the above ink jet technique for coating of ink compositions.
It is also noted that various materials such as those known from patent publications 4 and 5 may be used for the light-emitting layer 23 and hole-injecting layer 22; no details of them are given herein.
After the hole-injecting layer 22 and light-emitting layer 23 have been formed in the hole 4 in the bank 33 in this order, a transparent electrode that provides the cathode 24 of the organic EL light emitter 2 is deposited all over the surface of the substrate by means of sputtering. For the material of this transparent electrode, for instance, use may be made of a calcium or other thin film having a low work function, on which there is additionally provided a gold or other thin film thin enough to transmit light satisfactorily.
In this way, the organic EL array 1 used in the embodiment of
It is noted that it is acceptable to use a thicker bank 33 having a deeper hole 4. In this case, the organic EL light emitter 2 is formed at the bottom of the hole 4, and a transparent material having a protective function or the transparent adhesive agent 13 is overlaid on the organic EL light emitter 2. Then, a ball lens 10 is fitted in the upper portion of that hole 4 so that it can be fixed in alignment with the light emitter 2.
How to use an organic EL array exposure head 101 according to one embodiment of the invention is now explained with reference to
Suppose now that such an organic EL array exposure head as mentioned above is used for, e.g., an exposure head of an electrophotographic type color imaging system.
Here, each organic EL array exposure head 101 (K, C, M, Y) comprises ball lenses 10 fixed onto organic EL emitters 2 in alignment therewith and in one-to-one relations thereto as shown in
For the developer 44 (K, C, M, Y), for instance, nonmagnetic one-component toner is used as the developing agent. The one-component developing agent is carried over, e.g., a feed roller to a developing roller. The thickness of a developing agent film deposited to the surface of the developing roller is controlled by means of a control blade, and the developing roller is brought in contact or engagement with the photosensitive member 41 (K, C, M, Y) to deposit the developing agent onto the photosensitive member 41 (K, C, M, Y) depending on the potential level thereof, so that toner images can be developed.
Black, cyan, magenta and yellow toner images formed at a station for forming four-color toner images for each color are sequentially primarily transferred onto the intermediate transfer roller 50 by means of a primary transfer bias applied to the primary transfer roller 45 (K, C, M, Y), at which they are superposed one upon another on the intermediate transfer belt 50, yielding a full-color toner image. This full-color toner image is secondarily transferred onto a recording medium P such as paper by means of a secondary transfer roller 66, and then passed between a pair of fixing rollers 61 to fix the toner image onto the recording medium P. Finally, the medium P is ejected onto a receiving tray 68 positioned on an upper site of the system by means of a pair of ejection rollers 62.
In
While the present invention has been described with reference to some preferred embodiments of the organic EL array exposure head, the imaging system incorporating the same, and the array-form exposure head fabrication process, it is understood that the invention is not limited to them and so various modifications may be made thereto. For instance, if a transparent conductive film is applied on the surface of the ball lens 10 to keep its surface potential equal to that of photosensitive drum 41 (K, C, M, Y), it is then possible to prevent electrostatic deposition of the developing agent or toner to the surface of the organic EL array exposure head of the invention, especially on the surface of the lens ball 10.
The present invention has favorable advantages over the prior art, as can be understood from the foregoing. According to the first organic EL array exposure head of the invention and the imaging system incorporating the same, an array of organic EL devices are arranged on a long length of substrate in at least one row of pixel pattern, and a ball lens is positioned in alignment with a light emitter of each organic El device on the light-emitting side of the array of organic EL devices, wherein the ball lens is located in such a way as to satisfy condition (8) where D is the distance between the surface of the light-emitting layer in the light emitter of each organic EL device and the apex of the entrance side-surface of the ball lens positioned in alignment with that light emitter and m is the diameter of the ball lens. Thus, nearly all diverging light leaving the light emitter of each organic EL device can be entered in the ball lens so that it can be utilized for pixel formation by projection; images of sufficient resolving power, brightness and contrast can be obtained at low power consumption yet with little or no crosstalk between adjacent pixels.
The third organic EL array exposure head of the invention (incorporated in the imaging system of the invention) comprises a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on the light-emitting side of the array of organic electroluminescent devices, wherein a space between adjacent ball lenses is filled with a fixing layer up to a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, wherein m is the diameter of said ball lens and n is the refractive index of said ball lens, so that said ball lens is fixed in place. Thus, the ball lenses can be firmly fixed onto the substrate with the light emitters located thereon without cutting off a light beam that leaves the light emitter and is projected onto a given surface to form a pixel, thereby obtaining a small-format organic EL array exposure head having sufficient mechanical strength for maintenance operations. By using a layer capable of absorbing light as that fixing layer, it is further possible to achieve an organic EL array exposure head that has reduced crosstalk between adjacent pixels and can give an image of sufficient resolving power, brightness and contrast at low power consumption.
In the fourth organic EL array exposure head of the invention (incorporated in the imaging system of the invention), the ball lens is at least embedded in an aerogel layer from a height m(1−1/n2) as measured from the apex of the entrance side-surface of said ball lens, where m is the diameter of said ball lens and n is the refractive index of said ball lens, up to a height at which the exit side-surface of said ball lens is overall covered with said aerogel layer. Thus, the ball lenses can be firmly fixed onto the substrate with the light emitters located thereon without cutting off a light beam that leaves the light emitter and is projected onto a given surface to form a pixel, thereby obtaining a small-format organic EL array exposure head having sufficient mechanical strength for maintenance operations. By providing a transparent protective layer, especially a transparent protective layer comprising a transparent conductive layer on the surface of the aerogel layer, it is possible to prevent fouling of the optical system of the organic EL array exposure head with toner or the like, which may otherwise lead to poor exposure or other defects.
In the above array-form exposure head fabrication process of the invention, a ball lens alignment mold is provided with an array of ball-receiving holes in an arrangement conforming to an arrangement of light emitters of said array of light emitting devices or secondary light sources, wherein each ball-receiving hole has a diameter equal to or slightly larger than that of said ball lens, ball lenses are fitted in said ball lens-receiving holes in said ball lens alignment mold, and each ball lens on said ball lens alignment mold is brought close to each light emitter of said array of light-emitting devices or secondary light sources while in alignment therewith, whereupon each ball lens is bonded to said array of light-emitting devices or secondary light sources. It is thus possible to make use of inexpensive, high-refractive-index ball lenses used for reflective paints or the like to fabricate an array-form exposure head such as an LED array exposure head at extremely low costs. In addition, it is possible to achieve an automated optical system alignment/fixation arrangement for high-quality, low-cost array-form exposure heads, thereby fabricating high-quality, low-cost array-form exposure heads.
Further, no large impact is likely to be applied on the ball lenses upon alignment, and so the ball lenses are unlikely to chip off to ensure provision of a high-quality array-form exposure head.
Furthermore, whether alignment is acceptable or unacceptable, and whether or not ball lenses chip off can be 100% inspected at high speed and with high precisions, so that high-quality, low-cost array-form exposure heads can be provided.
Claims
1. An organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on a light-emitting side of the array of organic electroluminescent devices, wherein:
- a space between adjacent ball lenses is filled with a fixing layer up to a height m(1−1/n2) as measured from an apex of an entrance side-surface of said ball lens, wherein m is a diameter of said ball lens and n is a refractive index of said ball lens, so that said ball lens is fixed in place.
2. The organic electroluminescent array exposure head according to claim 1, wherein said fixing layer comprises a layer capable of absorbing light.
3. The organic electroluminescent array exposure head according to claim 1, wherein said ball lens has a refractive index of 2 or greater.
4. An imaging system, which incorporates the organic electroluminescent array exposure head according to claim 1 as an exposure head for writing an image to an image carrier.
5. The imaging system according to claim 4, which is a tandem type color imaging system comprising at least two imaging stations, each comprising around the image carrier a charger means, an exposure head, a toner developer means and a transfer means, wherein color imaging is carried out by passing a transfer medium through each station.
6. An organic electroluminescent array exposure head, comprising a long length of substrate, an array of organic electroluminescent devices arranged on said substrate in at least one row of pixel pattern, and a ball lens positioned in alignment with a light emitter of each organic electroluminescent device on a light-emitting side of the array of organic electroluminescent devices, wherein:
- said ball lens is at least embedded in an aerogel layer from a height m(1−1/n2) as measured from an apex of an entrance side-surface of said ball lens, where m is a diameter of said ball lens and n is a refractive index of said ball lens, up to a height at which an exit side-surface of said ball lens is overall covered with said aerogel layer.
7. The organic electroluminescent array exposure head according to claim 6, wherein a transparent protective layer is provided on a surface of said aerogel layer.
8. The organic electroluminescent array exposure head according to claim 7, wherein said transparent protective layer comprises a transparent conductive layer.
9. The organic electroluminescent array exposure head according to any one of claims 6 to 8, wherein a fixing layer capable of absorbing light is provided on a substrate side of said aerogel layer.
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Type: Grant
Filed: Dec 16, 2003
Date of Patent: Oct 10, 2006
Patent Publication Number: 20040233271
Assignee: Seiko Epson Corporation (Tokyo)
Inventors: Shigeo Nojima (Nagano-Ken), Masatoshi Yonekubo (Nagano-Ken), Yujiro Nomura (Nagano-Ken)
Primary Examiner: Hai Pham
Attorney: Sughrue Mion, PLLC
Application Number: 10/736,235
International Classification: B41J 27/00 (20060101);